The technology of Coordinated Multipoint Transmission/Reception (CoMP) in the prior art refers to coordination between multiple geographically separate transmission points to lower interference between the transmission points to thereby improve the quality of signals received by users so as to improve effectively the capacity of a system and the spectrum efficiency of edge users. Typically the multiple separate transmission points refer to base station equipments of multiple cells or can be multiple Remote Radio Heads (RRHs) controlled by the same base station of a cell. With coordinated transmission between the multiple transmission points, interference between the different transmission points can be lowered and the throughput of users, particularly users at the edge of a cell, can be improved effectively. In the current study, a set of transmission points participating directly or indirectly coordinated transmission for User Equipment (UE) is referred to as a coordination set, and a set of transmission points for which the UE needs to measure downlink channel information is referred to as a measurement set, and a set of transmission points participating directly in transmitting a Physical Downlink Shared Channel (PDSCH) of the UE is referred to as a transmission set.
In a practical application, the technology of coordinated multipoint transmission/reception is generally categorized into Cooperative Scheduling/Beamforming (CS/CB) and Joint Processing (JP).
Cooperative scheduling refers to that respective base station performs coordination on time, frequency and space resources between cells to allocate orthogonal resources to different UEs to thereby avoid interference between the cells. Interference between the cells is a predominant factor restricting the performance of a UE at the edge of a cell, and with cooperative scheduling, interference between the cells can be lowered to thereby improve the performance of a UE at the edge of a cell. Referring to FIG. 1, for example, with cooperative scheduling of three cells, three UEs which may possibly interfere with each other are scheduled onto orthogonal resources, to thereby avoid interference between the cells effectively. Cooperative beam-forming refers to scheduling beam-forming directions of multiple users in cooperating cells, to make beam directions of different users in the cells mutually orthogonal to thereby lower interference between the users. Similar to Multi-User Multi-Input Multi-Output (MU-MIMO) transmission in a single cell, cooperative beam-forming is generally performed through interference suppression at a transmitter, in order to suppress the interference from transmission by the current user to transmission by another user, downlink channel information of the cooperating cells needs to be obtained at the base station end.
Joint processing includes two schemes of dynamic transmission point switching and joint transmission:
In the scheme of dynamic transmission point switching, a base station can switch dynamically a transmission point to transmit a signal, to a user to thereby select the optimum transmission point each time to transmit data; and in order to perform dynamic transmission point switching, the base station needs to obtain channel information of multiple optional transmission points over a feedback channel, or obtain channel information of the optimum transmission point.
In the scheme of joint transmission, the multiple transmission points transmit data concurrently to the UE to thereby enhance the received signal of the UE. As illustrated in FIG. 2, three transmission points transmit data to a UE on the same resource, and the UE receives the signals of the multiple transmission points concurrently; thus on one hand, useful signals from multiple transmission points may be superimposed to improve the quality of the received signal of the UE, and on the other hand, interference to the UE can be lowered to thereby improve the performance of the system. In this scheme, the UE is required to feed back channel information of the multiple transmission points and even relative channel information between the transmission points, to thereby enable the multiple transmission points to perform scheduling, pre-coding and data transmission jointly. This transmission scheme can be extended to a scheme of Multi-User Joint Transmission (MU-JT), that is, these multiple transmission points transmit data to multiple UEs concurrently on the same resource, and the performance of demodulation is ensured by orthogonal DMRSs between these multiple UEs.
In order to provide sufficient information for downlink CoMP transmission, the base station needs to indicate different types of reference signals or reference resources to the UE, and the UE performs some operations with these reference signals to support the different transmission schemes above. The reference signals or reference resources indicated by the base station may include but will not be limited to the followings:
1) Non-Zero Power Channel State Information Reference Signal (CSI-RS)
In order to measure channel state information of the multiple transmission points for CoMP transmission, the base station needs to indicate multiple non-zero power CSI-RS resources respectively for measuring Channel State Information (CSI) of the different transmission points. Generally each transmission point corresponds respectively to a non-zero power CSI-RS resource for measuring channel state information of the transmission point.
2) Zero-Power CSI-RS
For a UE (which is referred to as UE1 below), a transmission point other than its corresponding measurement set may also use a zero power CSI-RS for performing a channel measurement of a served UE (which is referred to as UE2 below). In order to enable these CSI-RS resources to be correctly received and used by the UE2 to measure CSI, the UE1 needs to perform rate matching for PDSCH data transmission at a corresponding location, that is, on a CSI-RS resource configured with zero power, PDSCH transmission of the UE1 may be muted so that the UE1 will not receive any PDSCH data transmitted by the base station side, and the base station will not transmit any PDSCH data to the UE1, all of which will be realized by configuring the zero power CSI-RS resource.
On the other hand, in some application scenarios, some resources may be configured for interference measurement, and zero power CSI-RSs will also be configured on these resources, to keep interference measurement from an influence of data transmission. The base station may configure the UE with multiple zero power CSI-RS resources for different use.
3) Interference Measurement Resource (IMR).
In order to enable the UE to measure a required interference signal, e.g., interference outside a certain set (measurement set or cooperation set) of transmission points, the base station will configure the UE with some interference measurement reference resources so that the UE measures interference on these resources to thereby estimate a CQI.
In order to obtain different interference information, the base station may configure the UE with multiple interference measurement reference resources. In a practical application, the base station may select for the UE the interference measurement reference resources among the zero power CSI-RSs, that is, the zero power CSI-RS resources and the interference measurement reference resources may partially overlap.
All of the three types of reference signals/reference resources above are notified to the UE through higher-layer signaling.
In a practical application, for an interference measurement reference resource for measuring interference outside a certain set of transmission points, the transmission points in the set of transmission points shall not transmit PDSCH data to any UE on this interference measurement reference resource, whereas transmission points other than the set of transmission points can transmit PDSCH data to any UE on the interference measurement reference resource without performing rate matching, so that all the UEs configured with the interference measurement reference resource may measure interference other than the set of transmission points correctly.
However, an interference measurement reference resource is currently configured by configuring a zero power CSI-RS resource which requires quasi-static higher-layer signaling so that the UE will not receive PDSCH data on the corresponding resource (that is, performing PDSCH muting); but a PDSCH transmission point of the UE may vary dynamically so that the configuration of corresponding zero power CSI-RS may fail to match with the current PDSCH transmission point, for example, an IMR is configured for measuring interference outside some transmission point or points, whereas a transmission point currently transmitting PDSCH data to the UE is not a transmission point corresponding to the IMR above, so that the UE will still receive the PDSCH data at the location of the IMR, and consequently the UE may not measure the magnitude of interference of the target transmission point, thus degrading the accuracy of estimating a CQI.